The invention relates generally to battery backed, electronic memory where the battery prevents loss of data during momentary power interruptions, and specifically to a battery backed memory suitable for use with memory integrated circuits providing for a low-power “powerdown” mode of operation.
Industrial controllers are special purpose computers used for the control of industrial processes and the like. While executing a stored program, they read inputs from the controlled process and, according to the logic of a contained control program, provide outputs to the control process.
Industrial controllers differ from regular computers both in that they provide “real-time” control (i.e., control in which control outputs are produced predictably and rapidly in response to given control inputs) and in that they provide for extremely reliable operation. In this latter regard, the volatile memory used by the industrial controller is often backed up with a battery so that data needed for the control program is not lost during momentary loss of line power. Volatile memory is that which requires power to maintain its stored data.
Such “battery backed” memory, using a combination of static random access memory (SRAM) and a long life battery such as a lithium cell, is well known. The lithium cell provides high power density and long life, but is not rechargeable, and must be replaced periodically.
In current control applications, synchronous dynamic random access memory (SDRAM) may be preferred to SRAM because of its higher density, faster speed, and lower cost. Unfortunately, the amount of power needed for SDRAM is much greater than that needed for conventional SRAM devices. For this reason, many SDRAMs provide at least two operating modes: a normal operating mode and a “powerdown” mode in which the power consumption by the SDRAM is significantly reduced while preserving data integrity. The powerdown mode may be activated by writing particular values to control registers within the SDRAM. The SDRAM may be set to powerdown mode by a microprocessor executing a program when main power is lost and it is known that battery backup will be required.
Despite the use of SDRAM having a powerdown mode, SDRAM battery backed systems may be plagued by high battery replacement rates.